An antenna device that needs protection from an external environment, such as rain, wind, snow, and dust, is normally stored in a radome when put into operation. The radome protecting the antenna device is present in a propagation path of radio waves radiated from an antenna and therefore required to have a good transmitting property and a small amount of reflection. Patent Document 1 discloses a radome of a sandwich structure in which a core material is sandwiched between two skin materials. The skin materials are made of fiber-reinforced plastic (FRP) and the core material is made of urethane. It is described that the radome of Patent Document 1 reduces a transmittivity loss of radio waves caused by the radome by bonding a ¼ wavelength or ¾ wavelength-thick core material between the skin materials.
Patent Document 2 discloses a radome chiefly mounted on aircrafts and having a streamline shape to lessen air resistance. Patent Document 2 points out a problem that when an aircraft takes a low elevation angle, an angle yielded between a communication direction of an antenna and a normal direction to the radome wall surface becomes so large that a power loss increases. According to the radome of Patent Document 2, an antenna device formed of a multi-layer dielectric material is provided in contact with an inner wall of the radome of a sandwich structure in which a core material is sandwiched between skin materials to make the radome function as a kind of sandwich plate, so that a transmitting property to communication power is improved by cancelling out reflected waves from the radome wall.